A malignant tumor sheds cells which migrate to new tissues and create secondary tumors while a benign tumor does not generate secondary tumors. The process of generating secondary tumors is called metastasis and is a complex process in which tumor cells colonize sites distant from the primary tumor. Tumor metastasis remains the major cause of deaths in cancer patients, yet the molecular mechanisms underlying tumor cell dissemination are not clearly understood.
Metastasis is a multi-step process in which tumor cells must detach from the primary tumor, invade the cellular matrix, penetrate through blood vessels, thus enter the circulatory system (intravasate), arrest at a distant site, exit the blood stream (extravasate), and grow. See, e.g., G. L. Nicolson (1982) Biochim. Biophis. Acta. 695: 113-176; G. L. Nicolson and G. Poste (1983) In. Rev. Exp. Pathol. 25: 77-181; G. Poste and I. J. Fidler (1980) Nature 283: 139-145; and E. Roos (1984) Biochim. Biophis. Acta. 738: 263-284. Given the complexity of the process, it is thought that numerous genes mediate tumor cell metastasis. Indeed, the metastatic phenotype has been correlated with expression of a variety of proteins, including proteases, adhesion molecules, and the like. However, evidence that a given protein is directly involved in dissemination is often lacking, or difficult to prove. L. A. Liotta and W. Stetler-Stevenson (1989) J. Natl. Cancer Inst. 81: 556-557.
The human epidermoid carcinoma, HEp-3, provides a unique system that can be used to detect and characterize genes which effect metastatic dissemination. HEp-3 cells, propagated by serial passage on the chick chorioallantoic membrane (CAM), are both tumorigenic and spontaneously metastatic (T+M+). L. Ossowski and E. Reich (1980a) Cancer Res. 40: 2300-2309. However, when such cells are grown continuously in vitro, they readily form primary tumors, but progressively become non-metastatic (T+M−) with time. L. Ossowski and E. Reich (1980b) Cancer Res. 40: 2310-2315. With prolonged cultivation in vitro, they eventually become non-tumorigenic also (T−M−). The loss of metastatic ability is reversible. T+M− cells carried on the chorioallantoic membrane for two to three passages regain the ability to form spontaneous metastases. Thus, by altering growth conditions, the metastatic potential of these cells can be manipulated by the investigator.
Human urokinase-type plasminogen activator (uPA) was shown to be directly involved in dissemination of HEp-3, as spontaneous metastasis of HEp-3 cells in the chick embryo was inhibited by antibodies that were specific for human uPA. L. Ossowski and E. Reich (1983b) Cell 35: 611-619. Subsequently, it was observed that inhibition of uPA activity blocked infiltration of the CAM mesenchyme by individual HEp-3 cells. L. Ossowski (1988a) Cell 52: 321-328. However, active uPA appeared to be required for tumor cell intravasation but not extravasation. L. Ossowski (1988a). Thus, some other factor(s) must be also involved in HEp-3 dissemination and in dissemination of cancer cells in general. J. P. Quigley et al. (1988) Ciba Foundation Symposium 141: 22-47, Brooks et al. (1993) J. Cell Biol. 122 (6): 1351-1359 and Testa, et al. in U.S. Pat. Nos. 6,245,898 and 6,498,014 describe the generation of monoclonal antibodies (mAbs), using “subtractive immunization”, which recognize cell surface antigens expressed on HEp-3 cells and inhibit tumor metastasis in the chorioallantoic membrane model. Nevertheless, these antigens have not been correlated to in vivo metastasis nor shown to be directly involved in the process of in vivo metastasis.
Consequently, there is a need to identify biological molecules that are functionally involved in cancer cell dissemination in order to develop therapies that can be used to inhibit the migration of tumor cells to new tissues. Also, methods to inhibit tumor cell metastasis and to diagnose cancer are needed to help in the battle to control cancer by reducing or eliminating the spread of cancer cells throughout the body of a mammal afflicted with cancer.